1. Field of the Invention
The invention relates to a process for the production of infra red reflecting fluorine-containing tin oxide coatings on flat glass.
2. Description of the Prior Act
It is known that fluorine-containing tin oxide coatings on flat glass of appropriate thickness, for example 100 nm to 1000 nm, reflect infra red radiation whilst transmitting light in the visible part of the spectrum. It is also known to make part of the spectrum. It is also known to make such coatings by spraying an aqueous solution of tin chloride containing fluorine in the form of hydrofluoric acid or an ammonium fluoride on to hot glass. More recently, it has been proposed, in UK Pat No. 1,565,765, to overcome some of the practical problems encountered in operating a solution spray process by projecting an organic tin compound in the form of a finely divided powder suspended in a stream of carrier gas containing hydrogen fluoride onto the hot glass. A modified form of this process, which simplifies the problem of controlling the tin:fluorine ratio in the coating produced, is described in European patent application No. 39,256 A1; in this process, a powdered fluorine compound, dispersed in the carrier gas used for the organic tin compound, is used as a source of fluorine in place of the hydrogen fluoride gas. In a preferred form of the invention described in European patent application No. 39,256 A1, a single organic compound such as dibutyl tin difluoride is used as a source of both tin and fluorine.
The coatings produced in accordance with the teaching of European patent application No. 39,256 A1 have high infra red reflectivity and are generally colourless in appearance although, in common with other thin films, they give rise to interference colours in reflection. The interference colours depend on the thickness of the coating, so that variations in thickness across a coating lead to an observable variation in the interference colour seen. The visual impact of the interference colour, and the variations in such colour resulting from variations in thickness of the coating, are generally less the greater the mean coating thickness of the coating. However, it will be appreciated that, since the coatings absorb visible light, the light transmission of the coated glass is reduced as the thickness of the coating is increased.
We have proposed, in our copending UK patent application No. 2,156,386A, published after the priority date of this application, to increase the light transmission of the coatings of given thickness by using solid finely divided volatile inorganic tin (IV) compounds which contain both chlorine and fluorine in place of the organic tin compounds used in European patent application No. 39,256 A1.